Amorphous silicon deposition for sequential lateral solidification

ABSTRACT

An amorphous silicon deposition method includes the step of forming a buffer layer on a transparent substrate; depositing amorphous silicon on the buffer layer by a thickness ranging from 600 to 2,000 angstroms; repeatedly irradiating the amorphous silicon layer to completely melt using a laser beam that has a complete melting energy density; and moving the laser beam by a transaction distance for the next laser beam irradiation.

[0001] This application claims the benefit of Korean Patent Application No. 2001-29885, filed on May 30, 2001 in Korea, which is hereby incorporated by reference as it sully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an amorphous silicon deposition, and more particularly, to an amorphous silicon deposition method that is useful for sequential lateral solidification (SLS).

[0004] 2. Discussion of the Related Art

[0005] Flat panel display devices that are thin, lightweight, and have low power consumption are in high demand. Flat panel display devices may be classified into two basic types. One type is a light-emitting display device that emits light to display images, and the other is a light-receiving display device that uses external light source to display images. Plasma display panels (PDPs), field emission display (FED) devices, and electro luminescence display devices are examples of light-emitting displays. Liquid crystal displays are examples of light-receiving displays. The liquid crystal display is widely used for laptop computers and desktop monitors because of its superior resolution, color range, and image quality.

[0006] Liquid crystal display (LCD) device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce images. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.

[0007] While various types of liquid crystal display devices are known, active matrix liquid crystal display (AM-LCD) having thin film transistors (TFTs) and pixel electrodes arranged in a matrix are probably the most common. This is because an active matrix LCD can produce high quality images at reasonable cost.

[0008] The TFTs of LCDs generally include polycrystalline silicon (p-Si) or amorphous silicon (a-Si) as an active layer. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, it is more widely used for switching elements in liquid crystal display (LCD) devices. However, amorphous silicon (a-Si) has problems when used in large-area LCD devices because the electrical characteristics of amorphous silicon are problematic. In contrast to amorphous silicon, polycrystalline silicon provides faster display response times when used in TFT switching elements. Thus, polycrystalline silicon (p-Si) is better suited to large-area LCD devices, such as large laptop computers and television sets, which require greater field effect mobility.

[0009] During the formation of polycrystalline silicon for LCD applications, laser treatment techniques are frequently used. Such polycrystalline silicon also can be used in the driving circuits for the TFT switching devices.

[0010]FIG. 1 is a schematic diagram showing a main component of an active matrix liquid crystal display. In FIG. 1, the LCD device includes driving circuits 3 and an active matrix 4 on a substrate 2. The active matrix 4 is located in the central portion of the substrate 2, while the gate driving circuit 3a and the data driving circuit 3 b are respectively located in the left and top portions of the substrate 2. In the active matrix 4, a plurality of gate lines 6 are disposed in a transverse direction, and a plurality of data line 8 are disposed in a longitudinal direction (perpendicular to the gate lines 6). Pairs of gate and data lines 6 and 8 define a plurality of pixel regions. The gate driving circuit 3 a transfers address signals to the gate lines 6, and the data driving circuit 3 b transfers display signals to the data lines in synchronism with the address signals. A plurality of electrically independent pixel electrodes 10 are arranged in pixel regions. At intersections of gate lines 6 and data lines 8 are a plurality of thin film transistors (TFTs) “T” that act as a switching devices. The TFTs form a matrix format, and each TFT “T” corresponds to a particular pixel electrode 10.

[0011] The gate driving circuit 3a and the data driving circuit 3b are electrically connected to a control signal input port 12 that is interconnected to a controller (not shown) that controls the gate driving circuit 3 a and the data driving circuit 3 b. The gate driving circuit 3 a and the data driving circuit 3 b include CMOS (complementary metal oxide semiconductor) transistors that act as inverters that transfer signals to the pixel electrodes 10. Namely, the gate driving circuit 3 a and the data driving circuit 3 b are silicon thin film complementary metal oxide semiconductor (CMOS) structures having a shift register. The shift register can be a static or dynamic circuit of complementary (P type and N type) or monoconductive TFTs.

[0012] It is very important that an active matrix liquid crystal display (AM-LCD) device similar to FIG. 1 have an active matrix 4 and driving circuits 3 with low off state leakage currents. Additionally, it is important that the CMOS transistors have high field effect mobility. However, polycrystalline silicon used in TFT switches and CMOS transistors have many crystal grains, and thus grain boundaries. Those grains and boundaries interrupt carrier movement and cause deterioration of the active matrix elements. If the grains are larger and the grain boundaries more regularly distributed within the polycrystalline silicon, the field effect mobility increases. Thus, a silicon crystallization method that produces large grains is an important issue.

[0013] To solve the problems mentioned above, the crystal field distribution must be controlled, or a single crystal device is required. A technique of forming a single crystalline silicon layer on a glass substrate by sequential lateral solidification (SLS) is described in Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, 956-957, 1997. That technique uses the fact that silicon grains tend to grow laterally from the interface between liquid and solid silicon. Based on that principles the reference teaches crystallizing amorphous silicon layer by controlling the laser energy and irradiation range of a moving laser beam to produce silicon grains having predetermined length, as shown in the following example. Accordingly, sequential lateral solidification (SLS), which induces lateral growth of silicon grains, can form single-crystal silicon film using laser energy.

[0014]FIG. 2 is a graph illustrating grain size verses laser energy density, and FIGS. 3A-3C present cross-sectional views for explaining the mechanism of forming p-Si film composed of large grains. As shown in FIGS. 3A to 3C, a buffer layer 12 and an amorphous silicon layer 14 are sequentially formed on a transparent substrate 1.

[0015] Referring now to FIGS. 2 and 3A, the first regions is a partial melted regime. When a laser beam having an energy density within the first region irradiates the amorphous silicon layer 14, only a surface portion “A” of the amorphous silicon layer 14 is melted. Thereafter, during an annealing process a plurality of small grains “G1” are formed in a vertical direction.

[0016] The second region of FIG. 2 illustrates a laser beam having an energy density that nearly melts the complete region. When a laser beam having an energy density in the second region irradiates the amorphous silicon layer 14, almost all of the amorphous silicon is melted, reference FIG. 3B. Furthermore, a plurality of seeds 13 is formed between the amorphous silicon layer 14 and the buffer layer 12. Due to the seeds 13, the silicon grains tend to grow horizontally. However, since the plurality of seeds 13 are distributed randomly over the transparent substrate 1, it is difficult to obtain a plurality of uniform grains “G2” even though the grains “G2” are larger.

[0017] The third region of FIG. 2 illustrates a complete melting regime. When a laser beam having an energy density within the third region irradiates the amorphous silicon layer 14, all of the amorphous silicon is melted, reference FIG. 3 C. Then, homogeneous nucleation is conducted during the annealing process. Therefore, a plurality of nuclei 15 is formed in the melted silicon, and fine grains “G3” are obtained.

[0018] In view of above-mentioned silicon crystallization mechanism, suitable laser energy densities can be obtained experimentally, and the region of suitable laser energy densities can be referred as a process window.

[0019]FIG. 4 is schematically illustrates sequential lateral solidification (SLS). When a first laser beam having an energy density that completely melts the amorphous silicon layer 20 a first complete melting region “I” is induced. Furthermore, a plurality of seeds 24 are formed at interfaces 24 between the solid phase amorphous silicon and the liquid phase silicon due to the liquid-solid phase contact. During annealing, the plurality of seeds 24 tend to grow laterally (opposing vertical directions in FIG. 4), thereby generating a plurality of first grains 26. Furthermore, during annealing a plurality of fine grains 28 can be formed near the middle of the first melting region “I”, and thus the first grains 26 divided that region into two parts.

[0020] After forming the first grains 26, a second laser beam having an energy density that completely melts the silicon layer is applied. The second laser beam beneficially has the same energy density and beam width as the first laser beam. Before the second laser beam irradiates the substrate 22, it is moved such that the bottom area of the first melting region “I” (portion II) and the fine grains 28 will be irradiated. The second laser irradiation completely melts the portion “II” a second time and re-melts the first grains. During annealing the first grains 26 of the top area “III” tend to grow laterally until it forms a single crystal grains 30. Namely, the single crystal grains 30 are formed by repeating the first and second laser beam irradiation and annealing processes.

[0021] In the above-mentioned SLS process the distance between the first and second laser beam irradiations is beneficially the length of lateral growth of the first grains 26 in the top area “III”. That length is often referred to as a translation distance. By way of adjusting the translation distance, the fine grains 28 must be removed from the middle of the first laser beam width “I”.

[0022] For example, a laser beam having a beam width of 1 to 3 millimeters can be used and an overlapping ratio of 90% is suitable. The amorphous silicon layer is usually formed with a thickness of 500 angstroms. If the thickness of the amorphous silicon layer was greater than 500 angstroms, the laser beam would have to a greater laser density energy to melt the thicker amorphous silicon, which would increase manufacturing costs and possibly slow down the fabrication process, with no improvement in the characteristics of the thin film transistors being produced. Accordingly, when depositing the amorphous silicon to induce the polycrystalline silicon, the amorphous silicon layer should maintain a thickness ranging from 500 to 600 angstroms.

[0023] Since SLS crystallization uses a laser beam having an energy density sufficient to completely melt the silicon layer, the thickness of the amorphous silicon layer is an important issue. If the amorphous silicon layer has a thickness of about 500 angstroms, defects occur during SLS crystallization. When the translation distance is short defects such as a pilling phenomenon and an agglomeration phenomenon may occur and the range of the process window decreases.

[0024]FIG. 5 illustrates laser energy density during SLS crystallization when the amorphous silicon layer has a thickness of 500 angstroms. In SLS crystallization, when the laser energy density is about 410 mJ/cm² for the formation of single crystal silicon, the translation distance can range from 0.05 to 0.9 micrometers to compose the process window. Furthermore, when the laser energy density is about 450 mJ/cm², the translation distance ranges from 0.6 to 1.0 micrometers.

[0025] If the translation distance is more than 0.95 micrometers at a laser energy density of 425 mJ/cm², as shown in FIG. 5, resulting silicon becomes polycrystalline silicon instead of single crystalline silicon. On the other hand, if the translation distance is less than 0.3 micrometers at an laser energy density of 425 mJ/cm2, resulting silicon becomes crystalline silicon having dome defects, such as pilling and agglomeration. Accordingly, 500 angstroms thick amorphous silicon has a narrow process window and is susceptible to defects.

SUMMARY OF THE INVENTION

[0026] Accordingly, the present invention is directed to an amorphous silicon deposition method for sequential lateral solidification (SLS) that substantially obviates one or more of problems due to limitations and disadvantages of the related art.

[0027] An advantage of the present invention is to provide an amorphous silicon deposition method for sequential lateral solidification (SLS) that increases the process window.

[0028] Another advantage of the present invention is to provide an amorphous silicon deposition method for sequential lateral solidification (SLS) that increases the manufacturing yield and the product stability.

[0029] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the method particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0030] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an amorphous silicon deposition method for sequential lateral solidification (SLS) includes the step of forming a buffer layer on a transparent substrate; depositing amorphous silicon on the buffer layer at a thickness ranging from 600 to 2,000 angstroms (beneficially 600 to 900 angstroms); repeatedly irradiating the amorphous silicon layer so as to completely melt that layer using a laser beam that has a complete melting energy density; and moving the laser beam by a translation distance for the next laser beam irradiation.

[0031] The above-mentioned method further includes annealing the molten silicon layer to form a single crystalline silicon layer after each laser beam irradiation. Laser energy density ranges from 310 to 515 mJ/cm² are suitable. When the laser beam has an energy density ranging from 310 to 313 mJ/cm², the maximum translation distance ranges from 0.2 to 1.0 micrometers and the minimum translation distance ranges from 0.04 to 0.1 micrometers. Beneficially, the amorphous silicon layer deposited on the buffer layer is 1,000 angstroms thick.

[0032] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention.

[0034] In the drawings:

[0035]FIG. 1 is a schematic block diagram showing the main component of an active matrix liquid crystal display;

[0036]FIG. 2 is a graph showing grain size verses laser energy density;

[0037] FIGS. 3A-3C are cross-sectional views illustrating the crystallization of a-Si film using SLS;

[0038]FIG. 4 is process drawings illustrating sequential lateral solidification (SLS);

[0039]FIG. 5 is a graph showing laser energy densities that can frame a process window during SLS crystallization when the amorphous silicon layer has a thickness of 500 angstroms;

[0040]FIG. 6 is a graph showing a laser energy density that can frame a process window during SLS crystallization when the amorphous silicon layer has a thickness of 1,000 angstroms; and

[0041]FIG. 7 is a graph showing a transfer characteristics comparison between the conventional art and the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0042] Reference will now be made in detail to illustrated embodiments of the present invention, examples of which are shown in the accompanying drawings. Wherever possible, similar reference numbers will be used throughout the drawings to refer to the same or similar parts.

[0043]FIG. 6 is a graph showing a laser energy density that can frame a process window during SLS crystallization when the amorphous silicon layer has a thickness of 1,000 angstroms according to the present invention. When the laser energy density is 310 mJ/cm², the maximum translation distance is 0.2 micrometers and the minimum translation distance is 0.05 micrometers. Laser energy density of 415 mJ/cm² corresponds to a maximum translation distance of 0.6 and a minimum translation distance of 0.04 micrometers, the laser energy density of 450 mJ/cm² corresponds to a maximum translation distance of 0.8 and a minimum of translation distance 0.04 micrometers, and a laser energy density of 515 mJ/cm² corresponds to a maximum translation distance of 1.0 and to a minimum translation distance of 0.1 micrometers.

[0044] Compared with FIG. 5, FIG. 6 shows a much larger process window. Namely, since the amorphous silicon layer is deposited at a thickness of 1,000 angstroms (larger than the conventional art) the process window is wider range. Furthermore, defects, such as pilling and agglomeration, occur at a short translation distance than in the conventional art. For example, when a laser beam having an energy density of 415 mJ/cm² irradiates the silicon layer to form single crystalline silicon, the defects occur below a transaction distance of 0.04 micrometers. Accordingly, the width of the process window increases. Furthermore, although the amorphous silicon is larger than in the conventional art, the energy consumption of the laser beam does not greatly increase.

[0045]FIG. 7 is a graph showing a transfer characteristic comparison between the conventional art and the present invention. As the gate voltage V_(g) was gradually increased from −15V to +20V at a fixed rate, the variation of the drain current I_(d) flowing through an active channel layer was measured. Further, the drain voltage V_(d) was set at 0.1V and 10V (V_(d)=0.1V and V_(d)=10V), while the transfer characteristics are measured. The amorphous silicon layer of the conventional art was 500 angstroms thick while the amorphous silicon layer of the present invention was 1,000 angstroms thick.

[0046] As shown in FIG. 7, when the drain voltage V_(d) was 0.1V, the field effect mobility of the thin film transistor was 230 cm²/V·s using conventional silicon of 500 angstroms thick. On the contrary, when the drain voltage V_(d) was 10V, the field effect mobility of the thin film transistor was 390 cm²/V·s in the inventive silicon of 1,000 angstroms thick. Moreover, when the drain voltage V_(d) was 10V, it was shown that the leakage current was lower in the thin film transistor having the inventive silicon layer rather. Therefore, the thin film transistor having an active layer made from amorphous silicon layer that is 1,000 angstroms thick can have improved electrical characteristics. As a result of the experiment shown in FIG. 7, the amorphous silicon layer should be between 600 to 2,000 angstroms thick according to the present invention when the amorphous silicon is crystallized by SLS and used as an active layer of the thin film transistor.

[0047] As described above, if single crystalline silicon layer is formed from amorphous silicon depositions in accord with the present invention, the field effect mobility of the thin film transistor is improved, and thus the thin film transistor is suitable for use in liquid crystal displays having high resolution. Namely, the silicon layer of the present invention can be employed as elements of CMOS transistors that are in the driving circuits of a liquid crystal display having excellent image quality.

[0048] It will be apparent to those skilled in the art that various modifications and variations can be made in the method of crystallizing the amorphous silicon without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A crystallization method, comprising the step of: forming a buffer layer on a transparent substrate; depositing a 600 to 900 angstrom thick amorphous silicon layer on the buffer layer; repeatedly irradiating the amorphous silicon layer using a laser beam that completely melts a region of the amorphous silicon layer; and moving the laser beam a translation distance between irradiations.
 2. The method according to claim 1, further comprising annealing the silicon layer between irradiations.
 3. The method according to claim 1, wherein the laser beam has an energy density between 310 and 515 mJ/cm².
 4. The method according to claim 1, wherein the maximum translation distance ranges from 0.2 to 1.0 micrometers.
 5. The method according to claim 1, wherein the minimum translation distance ranges from 0.04 to 0.1 micrometers.
 6. The method according to claim 1, wherein the amorphous silicon layer deposited on the buffer layer is 1,000 angstroms thick. 